The demand for batteries with high energy capacity, low weight and long lifetime has become increasingly important in a variety of fields and industries, including those relating to portable electronic devices, electric vehicles, and implantable medical devices. For example, the energy capacity, weight and cycle life characteristics are often useful for improving the functionality of a particular device in which the batteries are used. In portable electronic devices and implantable medical devices, these and other related aspects are useful to allow for increases in power (e.g., from additional processing power) and/or reduction in the size of the devices. In electric vehicles, these aspects are often limiting factors in the speed, power and operational range of the electric vehicles.
Various commercial embodiments of batteries function as an electrochemical cell that stores and converts chemical energy from chemical oxidation and reduction reactions into a useable electrical form. The chemical reactions occur in the materials composing the two electrodes of the battery, such as reduction occurring in the cathode and oxidation occurring in the anode. These reactions are due in part to a difference in electrochemical potential between the materials comprising the anode and cathode. In many ion-based batteries, the two materials electrodes are separated by an ionic conductor, such as an electrolyte, that is otherwise electrically insulating. Each electrode material is electrically connected to an electronically conducting, preferably metallic, material sometimes called the current collector. The current collectors can then be connected to one another using an external circuit that allows for electron transfer therebetween. To equalize the potential difference, the anode releases ions (e.g., by oxidizing to form the ions) when electrons are allowed to flow through the external circuit. The flow of electrons is balanced by the flow of ions through the electrolyte. The ions then react with the chemically reactive material of the cathode. The number of ions that a material can accept is known as the specific capacity of that material. Battery electrode materials are often defined in terms of the energy capacity per weight, for example in mAh/g. Much research has been devoted to creating and developing higher energy density electrode materials for higher capacity batteries.
A specific type of battery is a Lithium-ion battery, or Li-ion battery. Li-ion batteries transport Li ions between electrodes to effect charge and discharge states in the battery. One type of electrode uses graphite as the anode. Graphite anodes have reversible (rechargeable) capacities that are on the order of 372 mAh/g. Graphite anodes function by intercalation of Li ions between the layered-structure. A limitation in some graphitic anodes is that Li is saturated in graphite at the LiC6 stoichiometry. Materials that can allow for larger amounts of Li insertion, therefore, have been attractive for use as high capacity Li battery anodes.
Some alternatives to graphite anodes utilize storage mechanisms that do not involve the intercalation of Li ions between layered-structure materials. For example, some transition metal oxides use a conversion mechanism that can provide relatively high energy anodes of 700 mAh/g. Other alternatives include elements, such as Si, Sn, Bi, and Al, which form alloys with Li through Li insertion. Some of these elements provide relatively large theoretical energy capacities. Often such elements exhibit a volume change during Li insertion. For example, pure Si has a theoretical capacity of 4200 mAh/g for Li4.4Si, but has been shown to produce as much as a 400% volume change during Li insertion (alloying). In films and micron-sized particles, such volume changes may cause the Si to pulverize and lose contact with the current collector, resulting in capacity fading and short battery lifetime. Electrodes made of thin amorphous Si may exhibit improvements in capacity stability over many cycles, but such films seldom have enough active material for a viable battery. Attempts to increase conductivity using conducting carbon additives have not completely solved such problems, since upon dealloying (delithiation), the particles may contract, and thereby, lose contact with the carbon. Si anodes have been prepared with a polymer binder such as poly(vinylidene fluoride) (PVDF) to attempt to hold the particles together, but the elasticity properties of PVDF may not be sufficient for the large Si volume change and do not completely mitigate the poor conductivity. This results in a low coulombic efficiency and poor cyclability. For example, the use of 10 μm sized Si particles mixed with carbon black and PVDF has been shown to result in a first discharge capacity of 3260 mAh/g; however, the charge capacity is only 1170 mAh/g, indicating a poor coulombic efficiency of only 35%. After 10 cycles, the capacity also faded to 94%. Moreover, conductive additives and binders add weight to the electrode, lowering the overall gravimetric and volumetric capacities of the battery.
These and other characteristics have been challenging to the design, manufacture and use of Li-alloy materials in Li-battery anodes. A solution has been to use nanostructure battery electrode materials. Nanomaterials include nanowires, nanoparticles, and nanotubes, all of which have at least one dimension in the nanometer dimension. Nanomaterials have been of interest for use in Li batteries because they have better accommodation of strain, higher interfacial contact area with the electrolyte, and short path lengths for electron transport. These characteristics may lead to improved cyclability, higher power rates, and improved capacity. Current efforts, however, leave room for improvement.